We have characterized the in vitro and in vivo characteristics of human adipose-derived mesenchymal stem cells (A-MSC), and compared them to marrow MSC (BM-MSC). We learned that MSC lodge at low levels into multiple tissues in immune deficient mice, but that both BM-MSC and A-MSC migrate from the bloodstream into areas of hypoxic tissue damage much more robustly, and we are continuing to define the mechanisms involved. We have dissected putative markers and phenotypes, and learned that surface proteins are altered dependent upon microenvironmental factors such as oxygen concentration and ligand binding. Our hypothesis is that the "tissue-repairing subset" of MSC is composed of highly adaptable cells that are poised to respond rapidly to the wound or inflammatory microenvironment by altering cell surface receptor expression, integrins, MMPS, and other proteins related to migration, retention, and survival. MSC are "paramedics", navigating through tissue to secrete paracrine factors that initiate cascades of endogenous repair and revascularization. It is beneficial if they are able to respond to the environment and to adapt very rapidly. But, this quality makes it difficult to prospectively isolate MSC for tissue repair. Thus, the phenotype of the most primitive MSC compartment, to allow prospective isolation from marrow or adipose tissue, is still not well defined and remains a goal of the grant application. We hypothesize that standard MSC culture conditions, which are non-physiological, are differentiating MSC to states that are not compatible with maximal tissue repair capacity. The goal of the current grant, reduced in scope is to subfractionate and to continue to examine MSC phenotype, in standard culture and in more physiological conditions, with the goal of prospectively isolating and potentially expanding primitive populations which retain the ability to form cartilage, bone, and fat and/or to home to damaged tissues. We will compare the phenotype and function of MSC expanded under standard conditions to more physiologic conditions, in comparison to prospectively isolated ALDH hi/CD45-/CD31-MSC populations. We will use clonal integration marking and novel xenotransplantation models to define conditions that allow recruitment of the highest numbers of primitive human MSC to areas of tissue damage, and to better define multipotency and self-renewal in this interesting adult stem cell population.